The synthesis method for metal-metal oxide in the prior art, such as the one disclosed in [Int. J Hydrogen Ener. 27, 1029 (2002)] by Toyota of Japan, uses trans-Diaamminedinitroplatinum (Pt(NO2)2(NH3)2) solution, which is a complex compound, as its precursor and adopts a two-step sintering (250° C. for 5 hours and 450° C. for 2 hours) to synthesize Pt—LiCoO2. Because crystalline platinum does not exist by this process, the overall hydrolysis rate is inferior to Pt—LiCoO2 with crystalline platinum. Meanwhile, the precursor Pt(NO2)2(NH3)2 can possibly release NO and NO2 during sintering, causing pollution to the environment. Accordingly, the present invention adopts chloroplatinic acid (H2PtCl6), which a kind of metal salt, as the precursor to synthesize the catalyst Pt—LiCoO2 with crystalline platinum. Not only hydrolysis efficiency will be increased, but also toxic gas will not be produced. Thereby, the present method is very practical to industry.